Radio Frequency Identification, or RFID, memory tags are known in many different forms for different applications. However, they all have in common a non-volatile memory, which in use stores data, and a transponder including an antenna coil for (wireless) inductive coupling with a transceiver. The memory tag is powered as a result of the inductive coupling, and is also read from or written to as a result of the inductive coupling. Different forms of RFID memory tag achieve the read/write communication in different ways, such as by amplitude modulation of the radio frequency signal, or by phase or frequency modulation. More detail of RFID memory tags can be obtained from the RFID Handbook, Klaus Finkenzeller, 1999, John Wiley & Sons.
Opto-electronic memory tags are also known, with one example being described in U.S. Pat. No. 6,299,068 B1. Such devices include a non-volatile memory which in use stores data and opto-electric cells which intercept light directed at the tag. The light powers the tag circuitry but is also modulated to provide data for writing to the tag memory and/or control signals to enable reading from the tag memory.
Using the same electromagnetic signals, whether radio frequency or light, for both supplying power and communication can be problematic, as the transmission of data or control signals can lead to inconsistent power supply, or consistent power supply can lead to inconsistent communication.